This invention relates to inspection apparatus, and more particularly, to inspection probes for inspecting tubular members.
There are many situations in which a hazardous environment limits human access to various locations. One such situation occurs in the inspection and repair of nuclear steam generators. A typical nuclear steam generator comprises a vertically oriented shell, a plurality of U-shaped tubes disposed in the shell so as to form a tube bundle, a tube sheet for supporting the tubes at the ends opposite the U-like curvature, and a dividing plate that cooperates with the tube sheet, forming a primary fluid inlet plenum at one end of the tube bundle and a primary fluid outlet plenum at the other end of the tube bundle. The primary fluid, having been heated by circulation through the nuclear reactor core, enters the steam generator through the primary fluid inlet plenum. From the primary fluid inlet plenum, the primary fluid flows upwardly through first openings in the U-tubes near the tube sheet which supports the tubes, through the U-tube curvature, downwardly through second openings in the U-tubes near the tube sheet, and into the primary fluid outlet plenum. At the same time, a secondary fluid, known as feedwater, is circulated around the U-tubes in heat transfer relationship therewith, thereby transferring heat from the primary fluid in the tubes to the secondary fluid surrounding the tubes, causing a portion of the secondary fluid to be converted to steam. Since the primary fluid contains radioactive particles and is isolated from the secondary fluid by the U-tube walls and tube sheet, it is important that the U-tubes and the tube sheet be maintained defect-free so that no breaks will occur in the U-tubes or in the welds between the U-tubes and the tube sheet, thus preventing contamination of the secondary fluid by the primary fluid.
Occasionally, it is necessary to either inspect or repair the U-tubes or the tube sheet welds by way of access through the primary fluid inlet and outlet plena. For this purpose, manways are provided in the vertical shell so that working personnel may enter the inlet and outlet plena to perform operations on the U-tubes and tube sheet. However, since the primary fluid, which is generally water, contains radioactive particles, the inlet and outlet plena become radioactive, which thereby limits the time that working personnel may be present therein. Accordingly, it would be advantageous to be able to perform operations on the U-tubes and tube sheet without requiring the presence of working personnel. There are several mechanisms known in the art that attempt to provide a solution to this problem; but none of them has been able to completely solve the problem.
A particular probe known in the art for inspection tubes has a single eddy current coil mounted on a mechanical support which constitutes the probe. The eddy current coil is mounted on a spring-loaded piston which has its axis on a radius of the probe so that the coil is pushed outwardly into contact with the internal surface of the tube and can follow the contour of the inner surface of the tube to be inspected. Since a single eddy current coil can detect a crack in only a 45.degree. sector of the tube wall, the single coil must be rotated to scan the entire surface of the tube wall. Since it is necessary to rotate this particular probe in order to inspect the entire tube at any particular cross-section, it has been found that rotation of the probe causes the coil to impact certain irregularities on the internal surface of the tube, which causes the eddy current coil to become damaged. Therefore, what is needed is a probe that is capable of remotely inspecting the internal surface of a tube in a nuclear steam generator without having to be rotated and that is capable of repeated use without being damaged.